1. Field of the Invention
The present invention relates to a method for the manufacture of a Fresnel lens sheet used in rear projection type image display devices and so forth and to a method for the manufacture of a molding die for a Fresnel lens sheet.
2. Description of the Prior Art
A schematic structure of a transmission type screen that has been typically used in rear projection type televisions is shown in FIG. 2. In FIG. 2, the reference numeral 1 stands for a Fresnel lens sheet and the reference numeral 2 stands for a lenticular lens sheet. The Fresnel lens sheet 1 and lenticular lens sheet 2 are usually tightly bonded to each other forming a transmission screen. The Fresnel lens sheet typically comprises a sheet having a Fresnel lens provided on the light outgoing surface thereof, the Fresnel lens being composed of equidistantly located, concentric circular lenses of a very small pitch. The Fresnel lens has a saw-tooth cross section with Fresnel lens surfaces 4 inclined with respect to a light incidence surface 3 (FIG. 3). In the lenticular lens sheet 2, semicylindrical lenses are arranged equidistantly on the light incidence surface and on the light outgoing surface. A parallel or converging light outgoing from the Fresnel lens sheet is greatly diffuses in the horizontal direction by the lenticular lens sheet 2. As a result, an image can be observed in a wide view range in the horizontal direction. A material containing a diffusing agent dispersed therein is typically used for the lenticular lens sheet to expand the image viewable range not only in the horizontal direction, but also in the vertical direction.
An image light L1 projected by a projector (not shown in the figures) disposed at the rear surface side of the transmission screen comprising the Fresnel lens sheet 1 and lenticular lens sheet 2 is refracted (L2) on the incidence surface 3 of Fresnel lens sheet 1, as shown in FIG. 3, again refracted on the outgoing surfaces (Fresnel lens surfaces) 4, and outgoes (L3) thus being converted into a parallel beam or a converged beam focused in a point located at a distance of 5xcx9c20 m to the observation side from the screen. However, when the light outgoes from the Fresnel lens surfaces 4, part of light L4 is reflected by the Fresnel lens surfaces 4, becomes confined in the Fresnel lens sheet, again reflected (L5) by the incidence surface 3, and again falls on the Fresnel lens surfaces 4. Such useless light beam L5 reaches the rise surfaces 5 directly or upon another reflection by the Fresnel lens surfaces 4 and outgoes from the rise surfaces 5 (L6).
Thus, in the conventional Fresnel lens sheet, the incident light beam L1 outgoes not only as the light beam L3 which essentially has to be the outgoing light beam, but also as the useless light beam L6 outgoing from a different place. As a result, a ghost image is produced and the contrast is greatly degraded. A technology of providing a light-absorbing layer or light-scattering layer on the rise surfaces is known for suppressing the formation of a ghost image by the useless light (Japanese Patent Application Laid-open Nos. S50-123448 and S52-143847).
Japanese Patent Application Laid-open No. S50-123448 discloses a technology by which a Fresnel lens sheet coated with a dispersion having suspended therein light-absorbing or light-scattering particles with a size of several microns to several tens of microns is rotated and a light-absorbing layer or light-scattering layer is formed on the rise surfaces by a centrifugal force. A problem associated with this publicly disclosed method was that when the viscosity of the dispersion that was be coated was low, the dispersion located on the peak portions of the rise surfaces was sprayed into the air by the centrifugal force at the time of spin coating, the film thickness decreased, and the desired light-absorbing layer or light-scattering layer was difficult to obtain. Another problem was that the dispersion bulged out toward the Fresnel lens surfaces in the valley portions of the rise surfaces and the transmissivity was decreased. Furthermore, Japanese Patent Application Laid-open No. S52-143847 disclosed a technology of coating the Fresnel lens sheet surface with a liquid prepared by mixing a fine powder of carbon black with an appropriate solvent and then wiping with a cloth to provide a light-absorbing layer. However, with such a technology, it was difficult to coat the liquid uniformly and only products with poor appearance could be obtained. In recent years, the pitch of Fresnel lenses is required to be decreased to improve the image contrast. However, in the Fresnel lens with a small pitch, the height of rise surfaces unavoidably decreases on the lens periphery. As a result, within the framework of the above-described conventional technology, it is difficult to provide on the rise surfaces a light-absorbing or light-diffusing layer which effectively suppresses ghost images.
In view of the foregoing, it is an object of the present invention to provide a Fresnel lens sheet which makes it possible to suppress the formation of ghost images by the useless light and to obtain images with good contrast.
It is another object of the present invention to provide a method for the manufacture of a molding die for such Fresnel lens sheet.
According to one aspect of the present invention, for achieving the above-mentioned object, there is provided a method for the manufacture of a Fresnel lens sheet having arranged thereon a multiplicity of sawtooth-like Fresnel lenses composed of a Fresnel lens surface demonstrating a convex lens function and a rise surface located between the adjacent Fresnel lens surfaces, this sheet having a layer containing a light-diffusing material or a light-absorbing material on the rise surfaces, the method comprising the steps of, coating a photoresist on the Fresnel lens surfaces and rise surfaces, and forming the layer containing a light-diffusing material or a light-absorbing material on the rise surfaces by exposing the photoresist at mutually different light exposure doses per unit surface area on the Fresnel lens surfaces and rise surfaces.
According to another aspect of the present invention, there is provided a method for e manufacture of a molding die suitable for the manufacture of a Fresnel lens sheet having arranged thereon a multiplicity of sawtooth-like Fresnel lenses composed of a Fresnel lens surface demonstrating a convex lens function and a rise surface located between the adjacent Fresnel lens surfaces, this sheet having a light-diffusing layer with a convexo-concave structure on the rise surfaces, the method comprising a step of forming a layer containing fine particles on the rise surfaces by coating a photoresist containing fine particles on the surface of a Fresnel lens original mold having Fresnel lens surfaces and rise surfaces or on the surface of a transfer mold of the Fresnel lens original mold and illuminating with light so as to obtain mutually different light exposure doses per unit surface area on the Fresnel lens surfaces and rise surfaces.